Verified in rhevm-3.2.4-0.44.el6ev.noarch (sf21).

Verification steps:
1) In older RHEVM 3.2 (3.2.0 aka sf17.5 in this case) create new VM with Windows OS type.
2) In DB, simulate the old behavior via seting the time_zone to empty string :
{{{
# psql -U engine engine
engine=> \pset null <NULL>
engine=> UPDATE vm_static SET time_zone = '' where vm_name = 'my-win7-sf17_5';
UPDATE 1
engine=> SELECT vm_guid, vm_name, time_zone from vm_static;
               vm_guid                |    vm_name     | time_zone 
--------------------------------------+----------------+-----------
 00000000-0000-0000-0000-000000000000 | Blank          | <NULL>
 993f9143-1a45-4f69-a739-34be9f62f049 | my-win7        | 
}}}
3) Run the VM in webadmin and try to edit for example the description -> you should get an error pop-up. Stop the VM.
4) Upgrade to RHEVM 3.2.4 (sf21)
5) Repeat step 3 -> now it should be possible to change the description.
6) Check the time_zone value in DB:
{{{
# psql -U engine engine
engine=> \pset null <NULL>
engine=> SELECT vm_guid, vm_name, time_zone from vm_static;
               vm_guid                |    vm_name     | time_zone 
--------------------------------------+----------------+-----------
 00000000-0000-0000-0000-000000000000 | Blank          | <NULL>
 993f9143-1a45-4f69-a739-34be9f62f049 | my-win7        | <NULL>
}}}

Since the problem described in this bug report should be
resolved in a recent advisory, it has been closed with a
resolution of ERRATA.

For information on the advisory, and where to find the updated
files, follow the link below.

If the solution does not work for you, open a new bug report.

http://rhn.redhat.com/errata/RHBA-2013-1468.html